Northern elephant seal pups had high mercury exposure in utero

In an announcement published Wednesday 04 November 2026, the US Geological Survey said researchers completed a 10-year collaborative effort to examine how maternal foraging strategies affect mercury transfer to northern elephant seal pups.

Researchers collected samples from pups and their mothers. Mercury transfer during gestation was related to mercury contamination in adult females and was strongly affected by the mothers’ foraging behavior. Fetal exposure also varied with the geographic location and depths where females foraged.

Mercury concentrations were highest in pups of females that made the deepest dives and foraged in the open ocean. Concentrations were strongly repeatable among successive pups of individual females. Females with lower-exposure foraging strategies generally had pups with lower mercury concentrations year after year.

Pups of the oldest females in the population had the highest mercury concentrations. A subset of females at the Año Nuevo colony, described as supermomms, represented 6 percent of females and produced 55 percent of all pups. The researchers said many of the most successful females may use foraging strategies that expose their offspring to the highest mercury concentrations.

Mercury exposure varied among years of the study. The highest pup concentrations occurred during years with a marine heatwave in the North Pacific. The agency said dietary shifts among foraging females or changes in mesopelagic mercury cycles could have affected exposure.

The study found that 70 percent of pups exceeded the established benchmark for aquatic mammals of 20 µg/g of dry hair. Mean concentrations were 23.01 µg/g, with a range of 8.03-63.09 µg/g and n=373. The agency said pups may face an elevated risk of sub-lethal adverse health effects and could serve as biosentinels for mercury contamination affecting other deep-ocean predators and their offspring.